Over the past two decades, palladium-catalyzed cross coupling technology has emerged as one of the most powerful tools in organic synthesis, both in academia and industry. Although either Pd(II) or Pd(0) in conjunction with a ligand can facilitate the cross coupling catalysis, it is well established that LnPd(0) (Ln=number of ligands) is the active catalytic species in the cycle. However during the initial step of reducing Pd(II) to Pd(0), there seems to be a lack of careful studies to understand the mechanism.
There is an increasing interest in using pre-formed Pd(0)(R3P)2 complexes as catalysts, instead of generating the catalyst in situ, as it avoids handling pyrophoric or air sensitive phosphine ligands, eliminates induction periods, and reduces undesired side products or sometimes no reaction (Grasa, G. A.; Colacot, T. J Org. Lett. 2007, 9, 5489). However, there are not many examples of Pd(0)(R3P)2 catalysts available commercially today, as their synthesis and scale up are tedious. A few methods are reported in the literature for the synthesis of Pd(0)(R3P)2 compounds. The method using Pd(η3-C3H5)(η5-C5H5) as a precursor (Yoshida, T.; Otsuka, S. Inorg. Synth. 1990, 28, 113) often suffers from its instability and volatility (Norton, D. M.; Mitchell, E. A.; Botros, N. R.; Jessop, P. G.; Baird, M. C. J. Org. Chem., 2009, 74, 6674); while the route involving Pd2(dba)3 (Hartwig, J. F. Organometallics 1995, 14, 3030) often requires recrystallization, which uses large amount of solvent under cryogenic conditions, therefore difficult to scale-up. More recently, Pd(η3-1-PhC3H4)(η5-C5H5) is reported (Norton, D. M.; Mitchell, E. A.; Botros, N. R.; Jessop, P. G.; Baird, M. C. J. Org. Chem., 2009, 74, 6674) to be a new precursor to synthesize L2Pd(0) in situ, although its scalability for industrial production is still unknown.
U.S. Pat. No. 4,196,135 describes the preparation of Pd[(o-tol)3]2. However, this method could not be reproduced. The major by-product was a black insoluble material, typical of reduced palladium and the starting palladium(II) complex. Hartwig et al. were also not able to reproduce this synthesis even by changing the reaction conditions (Hartwig, J. F. Organometallics 1995, 14, 3030).